Resistance device and insulator



Dec. 12, 1933. W

RESISTANCE DEVICE AND INSULATOR Filed May 31, 1929 I I 1 I I f/@ 2 lNvgNTOR W K4 A ONEY c. o. TERWILLIGER 1,938,674

Patented Dec. 12, 1933 UNITED STATES PATENT OFFICE RESISTANCE DEVICE AND INSULATOR Charles 0. Terwilliger, Mount Vernon, N.

assignor to Ward Leonard Electric Comp a corporation of New York This invention relates to improved resistance devices or resistance units and improved insulators or insulating material. It is particularly applicable to those forms of resistance devices in which the resistive conductor is wound or applied to a support of insulating material and covered with an insulating coating, such as vitreous enamel. The invention is also applicable where the apparatus or device including the insulating material is subjected during manufacture, orduring use, to comparatively high temperatures. For example, the improved insulating material is adapted to be moulded around metal portions of the device and then fired to form the insulator and support for the metal parts.

It has been customary in the manufacture of resistance units, in the form of tubular units, to use porcelain as the refractory tubular insulating support. On this tube the resistive conductor of the proper size and length is wound and attached to suitable terminals, which latter may be aflixed to the tube prior to the winding of the resistive conductor, or after the winding of the conductor. The next operation is to apply an adhesive vitreous enamel coating to the tube over the conductor and terminals. This is accomplished by dipping the unit in the enamel material, or spraying the enamel material over the unit and conductor, after which the unit is heated or fired in a furnace to the necessary temperature to melt or mature the enamel to form a glazed surface over the unit, thus forming an insulating protective coating impenetrable to moisture. In order to mature the enamel, temperatures of from 1,000 to 1,200 degrees F. or more are required.

The usual units of this type are composed of materials having relative expansion and coefllcients of expansion as follows:

Porcelain 2.54.6 or .0000025 to .0000046 Resistive conductor 14 or .0000140 Terminals 16 or .0000160 Enamel 9-11 or .0000090 to .000011 The first column of numerals in the above table indicates approximately the relative expansion of the different parts over the range of the usual enameling temperatures; and the second columnof numerals gives approximately the coeflicient of expansion per degree centigrade rise. The resistive conductor is assumed to be of nickel chromium, or of similar resistance conducting material, commonly used.

It is evident that in firing the unit for maturing the enamel, the resistive conductor and terminals tend to become loose on the refractory tube by reason of their much greater expansion. This sometimes results in, a shifting of portions of the conductor relative to other portions; and 50 when the turns are wound quite close together, they sometimes shift to such an extent that they touch each other at different points and form short-circuits. This causes a lack of uniformity in the resistance of the units when finished. When the variations are of sufilcient importance, they cannot be used for the purpose intended, requiring their rejection. Another diillculty arises on account of the unequal expansion of the parts due to the enamel flowing into the space between the resistive conductor and the refractory tube when the enamel has become softened during the maturing operation. The greater expansion of the conductor with reference to the porcelain body causes the turns of the conductor to sag and allow the softened enamel to pass betwen the turns of the conductor and the tube. Then when the units are cooled, after the maturing operation, the resistive conductor is placed under considerable strain, and in the case of fine conductors, they may actually break in one'or more places and cause an open circuit in the resulting unit.

Another trouble difficult to overcome arises in the adhesive vitreous enamel coating itself due to the different expansions of the parts composing such resistance units. This dimculty is the development, unless extreme care be taken in the manufacture, of innumerable cracks called crazing. This crazing sometimes permits moisture to reach the coated conductor and in time deteriorates the same and sometimes is sufiiciently present to cause the unit to have an unsightly appearance, instead of a smooth glazed surface.

These and other diillculties have required very great care in the manufacture of tubular resistance units, and also units of other forms, such as plate or disk forms, causing increase in time and cost of manufacture and constant inspection of the work as it progresses, as well as careful testing of the final product.

Recently the demand for high resistance units occupying comparatively small space has made it necessary to use an extremely small size of resistive conductor and to also wind the conductor on the tube to a very close pitch. Thus the difllculties due to the different expansion of the parts become particularly serious in the manufacture of such small high resistance units. 1.10

The main object of this invention is to produce an insulating body which will-not only have high insulating qualities comparable to that of porcelain, but will have a coefficient of expansion within the range of temperatures to which the body is to be subjected, that will substantially correspond or more nearly correspond to the expansion of the metal portions and other parts with which the insulating body is united, and thereby avoid the difficulties above referred to.

Another object is to produce an improved insulating refractory body, which requires less time of firing than that of the usual porcelains.

Another object is to produce an insulating body which will haveinsulating properties equal to, or superior to, that of porcelain and have equal or greater strength and toughness than that of porcelain. Another object is to produce an insulating material not only having substantially the same expansion as the resistive conductor, or other metal parts,'but one which will also withstand the sudden expansion and contraction without cracking or breaking during the manufacture thereof and during the firing to form the enamel coating. Other objects and advantages of this invention will be understood from the following description and accompanying drawing. 1

Fig. 1 is a side view of an illustrative resistance unit to which the invention is applicable and showing the unit before the enamel coating is applied; Fig. 2 is a longitudinal cross-section thereof with the enamel applied; Fig. 3 is a plan view of another form of illustrative resistance device to which the invention is applicable; and Fig. 4 is a cross-section showing the improved insulating material moulded about metal parts.

Referring to Figs. 1 and 2, the improved insulating body 10 is shown in the form of a tube and has wound around the same the resistive conductor 11. The latter has connected at its ends two terminal bands 12. The enamel coating 13 is applied to cover the resistive conductor and terminal bands, the enamel material first being applied by dipping the unit therein, by sifting the same thereon, or by spraying, or otherwise; and the unit is then placed in a furnace and heated to a sufficiently high temperature to form the vitreous adhesive insulating enamel coating.

Fig. 3 shows another form of resistance unit or device wherein the improved insulating body is in the form of a disk with an upturned circular edge 14. The resistive conductor 11 is applied thereto by positioning the same on the face thereof, usually in zig-zag form and has its ends connected to terminals 15 mounted in the upturned rim of the insulating body. An enamel coating 13' is applied to cover and adhesively vhold. the resistive conductor in position upon the insulating body. Various other forms of resistance devices or resistance units may embody my -invention and the illustrations are of known types to which the present invention is applied. Fig. 4 illustrates an example of the use of this improved material where it is moulded about parts before firing, and where the parts may be metals of a character capable of withstanding the heat of the maturing temperatures. The improved insulating material 16 is shown moulded about metal terminal posts 17, and in the material, holes 18 are formed before firing adapted for securing the terminal block to a support. It is evident that if the parts forming a resistance unit; or other devices, having substantially the same coefllcient of expansion over the range of temperatures to which they are subjected when combined, during manufacture, the difllculties already explained are overcome. In accordance with the present invention, the insulating body is one which, in addition to having high insulating properties, toughness and strength, also has substantially the same coeflicient of expansion as that of the part or parts with which it is combined, The improved material will alsowithstand the sudden expansion and contraction during the enameling without breakage. It also has a much greater maturing range than conventional porcelains, permitting the temperature during the firing of the insulating body to extend over a wider range without danger of making the finished body either overor under-matured.

If the expansion of the insulating body be made substantially the same as that of the resistive conductor, it is evidently desirable that the enamel be made of material having the same coefficient of expansion. This may be readily accomplished by selecting the materials forming the enamel of such ingredients and in such proportions as to give the desired degree of expansion, as is understood by those skilled in the art. For example, if a nickel chromium conductor be used having an expansion of say 14 during the maturing of the enamel, then a suitable enamel having substantially the same relative expansion may be formed from the following materials: feldspar 60 lbs., borax (Na2O.B2O3.10H2O) 42 lbs., carbonate of soda (NazCOal 15 lbs., saltpeter (/KNOa) 2 lbs. and cobalt oxide (000,00203) With reference to the refractory insulating I body, I have found that an insulating body fulfilling the above requirements and overcoming the above explained difficulties, may be made having talc (H2O.3MgO.4SiO2) as a principal ingredient, as it has desirable qualities and may be combined with other materials for securing the desired coefiicient of expansion in the resultant product. I have discovered that alkaline metal phosphates and alkaline earth metal phosphates which have a coefilcient of expansion 25 higher than that of talc and which do not disintegrate at the maturing temperature of the refractory material, may be very advantageously combined with talc for securing the desired results. An alkaline metal phosphate, that is a phosphate of an alkali-forming metal is a phosphate the primary element of which is an alkali metal, or an alkaline earth metal. One example of an alkaline earth phosphate which excellently fulfills the requirements is apatite [Ca4(CaF) (P04): or CadCaCl) (P0031; and an example of an alkaline metal phosphate, containing an alkali metal, is amblygonite [Li(AlF)P04 which fulfills the requirements. Instead of using apatite, other minerals of the apatite group may be used, and instead of amblygonite, other minerals of the amblygonite group may be used, which have a coemcient of expansion higher than that of talc and which do not disintegrate at the maturing temperature of the refractory material. In carrying the invention into practice other materials, such as clay and flint, may be added and produce an insulating body for reof between 26 to 626 amounts of the diii'erent materials, the insulating body may be made to have the particular coeflicient of expansion required.

For example, assuming that the expansion desired is that of a resistive conductor having a relative expansion of say 13, the insulating supporting body will have substantially the same expansion when containing approximately 44% talc, 17% apatite, 9% clay and 30% flint. Assuming the relative basis of expansion given in the foregoing table, talc has a coeflicient of expension of about 11.57 over the enameling range 0., apatite has a coeilicient of expansion of about 13.3 in the same enameling range, flint has a very high coeflicient of expansion of about 20.4 over the enameling range, and clay has a comparatively low coeflicient of expansion and depends upon the grade used. But, by combining these materials in the proportions above stated, the resulting relative expansion over the enameling range will be approximately 13.

In forming this improved insulating material, the talc, apatite, clay and flint is finely ground and mixed and then moulded or extruded, according to the usual practice followed with making porcelain material, to the desired form under pressure. It is then fired to mature it, as in the case of forming porcelain. Where the material is to be matured while united with the parts it is to support .or insulate, it is moulded about the parts, such as the metal contacts, terminals, or conducting portions and then matured. If the material be formed by extrusion, and metal parts are to be united with the insulating material before being flred, this is brought about by machining the formed material and inserting or attaching the metal parts.

To mature or fire the improved material, it is subjected to a temperature of 2000 F. to 2800 F., dependingmn its particular composition. The maturing or firing does not cause a fusing of the material, but a sintering takes place resulting in a strong refractory body of high insulating properties free from blisters or cracks. During the maturing, the material is merely brought to incipient vitrification, that is, partially vitrified and a homogeneous body is obtained more conveniently than in the making of porcelain and superior thereto in characteristics already described.

The usual process for maturing porcelain takes from 24 to 36 hours, but with-my improved material, one to three hours at the maturing temperature is usually suflicient.

It will be understood that the relative proportions may be varied in order to secure the particular character of improved insulating material desired, and that other materials may be added and some mentioned above may be omitted without departing from the scope of this invention; but, for best results, the mixture for forming the insulating material should have at least 10% talc and at least 5% apatite, or other alkaline metal phosphatefor minerals of the apatite group, or of the amblygonite group having the required characteristics of expansion and resistance to disintegration.

I claim:

1. The combination with a conducting metal element, of an insulator oi refractory material metal.

2. The combination with a conducting metal element, of an insulator of refractory material united therewith containing talc as a principal ingredient and an alkaline earth metal phosphate.

3. The combination with a conducting metal element, of an insulator of refractory material united therewith containing talc as a principal ingredient and a mineral of the apatite group.

4. The combination with a; conducting metal element, of an insulator of refractory material united therewith containing talc and apatite.

5. The combination with a conducting-metal element, of an insulator of refractory material united therewith containing talc, apatite, clay and flint.

6. The combination of an insulating refractory support, a metallic resistive conductor supported thereby, and an insulating fused coating covering said support and resistive conductor, said support containing talc as a principal ingredient and a phosphate of an alkali-forming metal.

7. The combination of an insulating refractory support, a metallic resistive conductor supported thereby, covering said support and resistive conductor,

said support containing talc as a principal in-.

gredient and an alkaline earth metal phosphate.

8. The combination of an insulating refractory support, a metallic-resistive conductor supported thereby, and an insulating fused coating covering said support and resistive conductor, said support containing talc as a principal ingredient and a mineral of the apatite group.

9. The combination of an insulating refractory support, a metallic resistive conductor sup and an insulating fused coating ported thereby, and an insulating fused coating covering said support and resistive conductor,

said support containing talc as a principal ingredient and a mineral of the amblygonite 10. The combination of an insulating refractory support, a metallic resistive conductor supported thereby, and an insulating fused coating covering said support and resistive conductor, said support containing talc and apatite.

11. The combination of an insulating refractory support, a metallic resistive conductor supported thereby, and an insulating fused coating covering said support and resistive conductor, said support containing talc and apatite, the amount of talc being greater than that of the amount of apatite.

12. The combination of an insulating retrace tory support, a metallic resistive conductor supported thereby, and an insulatingfused coating covering said support and resistive conductor, said support containing talc and apatite, the amount of the tale being at least 10% and of the apatite at least 5% of the mixture forming the support.

- 13. The combination of an insulating refractory support, a metallic resistive conductor supported thereby, and an insulating fused coating covering said support and resistive conductor,

said support containing talc and a phosphate of an alkali-forming metal having a coemcient of expansion greater than that of talc4nd which disintegrates only at a temperature higher than that of the maturing tem ra re o n d' 119- united therewith containing talc as a principal port. ingredient and a phosphate of an alkali-forming 

